1. Field of the Invention
This invention relates to apparatus and method for examination of a subject, such as a human body, by deriving a display representing subject structure from electrical signals produced by ultrasonic echoes within the subject which occur in response to the direction of an ultrasonic energy beam into the subject.
2. Description of the Prior Art
Systems are known for examining the internal structure of a subject, such as an animal body, by directing an incident ultrasonic energy beam into the subject, detecting effects of the beam within the subject, and producing from the detected effects a visual display indicating characteristics of the internal subject structure.
Such systems have included an ultrasonic transducer, and power circuitry for actuating the transducer to propagate an incident ultrasonic beam. Ultrasonic echoes occur within a subject when incident beam waves are propagated across interfaces within the subject. These interfaces are between tissue structures having differing acoustical characteristics, such as differences in density and acoustic velocity. The transducer included means responsive to the receipt of energy from these echoes to produce representative electrical signals. Some of the echo energy propagated back toward, and was detected by, the transducer.
Such systems further included display apparatus, and processing circuitry responsive to the electrical signals from the transducer to generate a visual image representing internal subject structure.
Prior art transducers have included an emitter element made of a piezoelectric substance which was responsive to the application of electrical voltage pulses to produce the incident ultrasonic beam. Conversely, the piezoelectric element was responsive to the receipt of ultrasonic echo energy to produce the electrical signals representing characteristics of the echoes.
Ultrasonic transducers have also typically included elements for coupling the transducer acoustically with the surface of the subject, to facilitate transmission of ultrasonic energy between the emitter and the subject. Such coupling elements have included a flat output surface for effectively establishing a uniform contact between the transducer and the subject surface. Coupling elements have sometimes been disposed to contact the emitter intimately, for optimum energy transfer to and from the emitter.
Transducer emitters having a flat disc-shaped configuration have been employed. The incident energy produced by the respective portions of a flat emitter proceed, unfocused, outwardly from the emitter along generally parallel paths.
An unfocused beam is normally inferior to a focused beam. It is often desirable to concentrate the incident energy on a particular portion of the subject, an operation that can most readily be accomplished if the ultrasonic incident energy is focused at a predetermined distance from the transducer.
To correct for this lack of emitter beam focus, it has been proposed to provide emitters having concavo-convex configuration, with the energy emitting face being concave. The ultrasonic energy emanating from the various portions of the concave emitter face converge toward a predetermined region located generally at a predetermined focal distance from the emitter. The focal distance of such an emitter is a function of the radius of curvature of the emitting surface.
Ultrasonic transducers have also included transformer elements for improving matching between differing characteristic acoustical impedances of the emitter and the subject. Improvement of this acoustical impedance matching enhances the efficiency of acoustical energy transfer to and from the subject by reducing energy losses due to acoustical reflections occurring when acoustical energy passes across interfaces between the emitter and the subject. Good impedance matching also reduces undesirable acoustical reverberations, or "ringing," within components of the transducer.
Every substance, or "medium" has a "characteristic acoustical impedance". The characteristic acoustical impedance of a medium is a function of the product of its density and the velocity at which acoustical waves propagate within the medium. For biological tissue, the acoustic velocity is substantially constant for each medium, regardless of acoustic frequency. Piezoelectric emitters generally have acoustical impedances of from 15 to 20 times greater than that of biological tissue, such as human body tissue.
When acoustical energy crosses an interface between two media having different acoustical impedances, echoes are reflected back from the interface. These echoes embody a depletion, or loss, of part of the energy of the incident acoustical wave. The greater the echo at an interface, the greater is the fraction of incident acoustical energy which is depleted from the wave which penetrates into the subsequent medium.
The occurrence of these echoes within the transducer structure itself is undesirable because it introduces spurious ultrasonic energy waves within the transducer. These spurious ultrasonic waves are translated by the transducer emitter into spurious electrical signals which interfere with desirable information bearing echoes representing signals from within the subject, and ultimately detract from the quality of the image derived from the processed electrical signals.
Acoustical impedance transformer elements have been used in association both with transducers having flat emitting surfaces, and with those having concave emitter surfaces for focusing the ultrasonic beam. The use of curved impedance transformers has been proposed for application to transducers with concave emitter surfaces.
When a flat impedance transformer was disposed across a concave transducer emitting face, excessive echoes and reverberations were generated within the transducer structure itself. These reverberations resulted from the fact that, in such a combination of emitter and transformer, there necessarily was a variation in thickness between the facing surfaces of the transformer and emitter. Because the thickness was not quarter wave matched over the entire surface unwanted echoes were generated. These echoes were undesirable because they caused the transducers to produce spurious electrical signals, interfering with signals generated from echoes emanating from within the subject.
To eliminate the problem of the void between the emitting face and the impedance transformer, it was proposed to provide an impedance transformer having a curved surface suitable for intimate contact with the curved emitting face, and a flat side opposite the emitting face for contacting the subject efficiently. A disadvantage of this transformer was that it was not uniform in thickness. It therefore did not, by definition, have uniform impedance matching characteristics as to all the acoustical energy of a given dominant frequency emanating from the emitting surface. The flat face of the transformer could not practicably be altered, (to give uniformity of thickness) because its planar character was necessary for efficient mechanical contact with the subject surface.
It has been proposed to employ a "quarter wave acoustical impedance transformer" element in the path of the ultrasonic beam generated by the emitter to effect a better impedance match between the emitter and the subject. A quarter wave acoustical impedance transformer has been embodied by a flat disc of material having a uniform thickness chosen to be approximately one-fourth of the wave length within the transformer of the dominant frequency of acoustical energy produced by the emitter. It is known that the interposition of a quarter wave transformer between a source of acoustical energy and a subject, or "load," receiving the acoustical energy, changes the effective acoustical impedance as seen by the source. More specifically, the interposition of the quarter wave transformer presents an acoustical impedance to the source which is a function of the square of the characteristic acoustical impedance of the transformer material divided by the characteristic acoustical impedance of the load. Therefore, a quarter wave transformer can be used to modify the terminating or "load" impedance as seen by the source. Proper choice of transformer thickness and material can thus be used to modify load impedance seen by the emitter to more closely "match" that of the emitter itself. The improved matching, as noted, reduces reverberations and facilitates energy material transfer to the subject.
Transformers having thicknesses of non-unity odd quarter wave length multiples have also been found useful in impedance matching.
Techniques and theory of acoustical impedance matching are explained in the following article, expressly incorporated by reference: Kossoff, G., Vol SU-13, IEEE Transactions on Sonics and Ultrasonics, pp. 20-30.